Internal Laptop SSD Drive Keeps Disappearing

My HP laptop has a second internal SATA SSD drive which I normally assign to drive Z: for backups. It has worked fine to date but sometimes when I start the PC it is not found in Explorer - see below. I can easily assign the disk letter Z: in Disk Management which works for a while but the drive still disappears after a while. I've enabled Automount in DISKPART. When I assign Z: I get a notification about what to do with removeable drives yet the SSD is not removeable. Perhaps Windows incorrectly thinks the drive is removeable?

What is going on here?

pjhutch said:

What backup software are you using? The backup software may have deliberately not assigned a letter to prevent the backsups from being corrupted, damaged in any way while it is 'online' This is good as ramsomware could potentially encrypt files on that drive if it was visible in explorer. Does having no drive letter prevent the backup from working?

I use Macrium Reflect Home version 8.0. This is recent problem during the past 2 weeks. Yes, the backup fails if there is no drive letter assigned which is usually how I spot the missing drive letter.

pjhutch said:

Have you run a chkdsk on the disk to check for corruption. To permanently assign a drive letter, open Computer Management console (compmgmt.msc), then use Disk Management to assign a letter to it (assuming nothing else is using it).

Chkdsk shows no problems. Disk management is the method I have been using to assign a disk letter but the disk keeps being disconnected.

pjhutch said:

Is there anything reported in the Event Logs?

No, nothing obvious