multi-boot Windows 10 install questions

Currently I have XP Pro 32-bit, XP X64, and Win 7 Pro 64 bit installed on my system. Each OS is installed on a separate hard drive. The system has 4 hard drives (and 5 partitions per drive) and I've setup the 4th hard drive with empty partitions in case I decide to buy and install Windows 10.

I assume I'd have to buy Win 10 since I'll be multi-booting instead of replacing Win 7.

I had to install Win 7 from XP X64 in order to specify the partition I wanted Win 7 to be installed on and to prevent it from re-lettering the partitions such as re-lettering the Win 7 partition to C: (it's installed on N: ).

If I do buy Win 10, will I need to go through the same install method, install from either XP X64 or Win 7 Pro 64 bit in order to specify the install partition for Win 10 and keep it from re-lettering partitions?

I'm also wondering about partition size. Currently I have Win 7 installed on a 64 GB partition, and I install almost all programs on a different 512 GB partition (O: ). Would 64 GB be enough for Win 10, and if not what partition size should I use?

10 takes a lot less space than 7 so a 64GB will be enough.
Can't help you with the other questions. I've never had a system with 4 hard drives.

Usually starting setup from within Windows keeps the drive letters as you have them, but it is not guaranteed. A full installation of Windows 10, Office and some other stuff should not exceed 30-40GB, so 64GB should be enough but then you have little space left for data. If the free space is less than 15GB you may also experience some delays due to insufficient space for the swap file.

Office would go into a separate partition. The swap file in yet another partition (on a different drive). I turn off hibernate mode so that hiberfil.sys doesn't take up space (that one can't be relocated).

The other issue is if I have to buy Win 10, or if I can use the Win 7 activation key to at least try Win 10, then remove it from my system (like doing an image restore of C: (boot) and N: (Win 7) partitions to get to a pre Win 10 state).

If you have a genuine Windows 7 with SP1 or Windows 8.1 installation, you can run a utility included in the Windows 10 setup and create a file GenuineTicket.xml Then clean-install Windows 10 (no need to upgrade first) and copy this file at the appropriate folder. Restart the computer with internet connection to activate. Read all about it here. So before you format the disk, create that file and save it on a USB Flash drive. Then you can just delete all partitions and clean-install Windows 10 and use the file to activate for free.

Thanks for the help, but I want to have both Win 7 and Win 10 installed and activated (separate partitions on separate drives), so to comply with Microsoft's rules, I'll have to buy Windows 10.

This is an old thread, but I just now determined a way to accomplish this. I had purchased a second copy of Win 7 Pro (originally was going to use this on the next system I buy), and it seems having two (purchased) instances of Win 7 Pro is the only solution to my original question. I again used XP X64 to install the second instance of Win 7 Pro (64 bit) onto the target partition, in which case Win 7 Pro retains the partition letter. I then ran Win 10 Pro setup from that second instance of Win 7 Pro to upgrade it, and the Win 10 retained the partition letter. I then removed the Win 7 image in windows.old using system file clean up.

In the case of installing Win 7 Pro from XP X64, there's a choice to either upgrade or to do a clean install to a partition selected by letter (in which case Win 7 will not change the partition letter). Win 10 install won't run from XP X64. When running Win 10 install from Win 7, there is no option for clean install, only to upgrade that instance of Win 7 to Win 10.

In summary, I needed two (purchased) instances of Win 7 in order to end up with a multi-boot (XP / XP X64 / Win 7 / Win 10) system without any partition letter changes.

As for the other part of my original question, I changed the Win 7 and Win 10 partition sizes to 96 GB, by taking 32 GB from the 512 GB "application / data" partitions reducing them to 480 GB.

cereberus said:

... it is totally NOT recommended to install XP on host pc these days as it has security holes like Swiss Cheese.

I rarely use the internet when running XP, so security isn't an issue. I also have backups to internal and external drives just in case. I use XP for old games and old utilities (mostly video processing / dvd creation - Adobe Premier Pro CS3). I wrote a backup / restore utility that copies folders and files along with security and owner info, which I can use instead of image backup for all partitions except Win 7 and Win 10 OS partitions. I use separate partitions for Win 7 and Win 10 data / applications and these can be backed up and restored with the backup utility, but it needs to run from XP or XP X64.

cereberus said:

Upgrade from Win 7 to Win 10 ... unstable

I only did this to preserve the partition letter. I selected "keep nothing" during the upgrade, and removed windows.old once Win 10 completed installation. I'm not aware of stability issues, especially if nothing (apps or data files) is carried over from Win 7 to Win 10.

cereberus said:

... it is irrelevant what drive letters you assign, as drive you boot to is always assigned "C" anyway.

On my system, Win 7 is running on partition "N" and Win 10 is running on partition "T". My prior post explains the install sequence needed to keep Win 7 or Win 10 from changing the partition to "C". For XP or XP X64, this isn't an issue, as they assign partition letters in a consistent matter, based on hard drive ordering in the BIOS. On my system, all 4 operating systems see the same partition letters.