Disk Management sees the disk and partitions - change drive letter greyed out?

Environment:

From: Windows XP-Pro x64

To: Windows 10 Pro

A user "left" the company and we found a Win XP-Pro x64 workstation under his desk - but no user/password for login.

I've taken the SATA drives (no RAID - JBOD) out of the XP box and put them into a Win 10 box to find out what's on them. But I can't access the 2nd disk.

Running Disk Management on Win 10 box:

• XP boot disk is good - see all partitions and able to assign drive letters.
• XP second disk - see all partitions, but unable to access any of them.
  • [Change Drive Letter] is greyed out for all three partitions on 2nd disk.

Am I missing something simple?