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Macrium Backup Fail
Error code 21
Random find:
Backup aborted! - Unable to read from disk - Error Code 21 - The device is not ready.
Assuming you are not using Macrium's boot disk (you provide minimal info) possibly some program is not enabling the backup to proceed.
See the discussion in that thread and feel free to search for others.
I was having troubles with Macrium Backup not completing I had a 1 button shortcut saved. Using the regular Macrium utility I found it was failing on a partition on the drive and aborting, unfortunately Windows Disk Management would not allow me to create a drive letter for that partition so I could not run file checks. For some days I had to change the backup to exclude that partition in order for Macrium to complete. Today I had right clicked on a drive and Properties, Tools, Optimize and Defragment, then selected Advanced and found all partitions on the hard drive shown and Optimized the hidden drives as well. 1 exception being a partion marked ESP which would not allow changes. Any way optimizing did it allowed all partitions to backup .
Another issue, though I back up to a 4TB drive, I have found I can not restore a bootable system drive from it while booting from a USB In order to restore I must first save the Macrium file to a 2TB SSD I have. Also seems necessary to change the uefi setting in system bios to boot from usb.
However if I only want to copy a saved file from a Macrium backup I can from the 4tb drive as long as I am careful to check mark the "enable access to restricted folders" in bottom left of page having double clicked to open a backup file.
One should also have a good USB 3 cable if using that older style connector as cables developed for USB1 and 2 may not seem to handle the speed.