Wild guess, but since they both crapped out at the same time while they were attached to the NUC, and then had to be repaired in Windows, my best bet would be a Linux kernel driver error.
Before RoonOS build 254 that was released last fall, it used the older Linux driver for NTFS. This one had some issues, mainly because NTFS is a proprietary and questionably documented file system that is very difficult to support fully. One of this was known to cause file system corruption by not properly enforcing writing only to completely error-free NTFS disks, and there was more.
Since RoonOS build 254, it uses the new NTFS driver that was added to the Linux kernel in spring 2022, made possible by a generous (and surprising) donation of code by Paragon Software, who had invested lots of efforts into reverse engineering NTFS.
This new driver is much better and much more strict about NTFS file system safety. After the build 254 RoonOS update, there were lots of cases on the forum where the new driver refused to mount disks that the older RoonOS had accepted.
The apparently most frequent cause of such refusal was unsafely removing (simply unplugging) the drive from a Windows machine (causing a “dirty unmount” NTFS flag), but I think there may have been others where they disks where never (at least knowingly) attached to Windows - just purchased (with NTFS preformatted) and attached to the NUC. At least that is my take from the many cases I have seen.
With the older driver it was definitely a bad idea to use NTFS with Linux for anything important, and the ROCK documentation specifically recommends against using it for external disks that are connected to ROCK. (https://help.roonlabs.com/portal/en/kb/articles/rock-storage-basics#USB_Storage_-_Best_Practice)
Even with the newer driver, and despite Paragon’s efforts, NTFS is still proprietary and reverse engineered, and it’s entirely possible that occasional errors occur without doing anything specific, simply caused by bugs in the Linux driver.
According to Danny, they are looking into a solution that avoids the necessary chkdsk after an unclean removal from Windows, but IMHO the advice to use a different file system remains valid, because the NTFS driver will always have deficiencies compared to an openly developed file system like Linux’s native EXT4 or a much simpler one like exFAT