Thanks for the replies.
I'm in the process of running chkdsk /r now -- I'll run it on all 6 partitions.
Well, I'm checking the Win10 partition (W:) and it is finding a lot of "bad clusters." I expected there to be problems since Win10 wouldn't even load, but do "bad clusters" indicate a physical problem?
Ok, the scan of W: is now complete, and to answer my own question, I presume that "bad clusters" are NOT a physical problem ("0KB bad sectors"). Here's the summary:
CHKDSK discovered free space marked as allocated in the
master file table (MFT) bitmap.
CHKDSK discovered free space marked as allocated in the volume bitmap.
Windows has made corrections to the file system.
122879999 KB total disk space.
26458040 KB in 96774 files.
84996 KB in 23263 indexes.
0 KB in bad sectors.
254427 KB in use by the system.
65536 KB occupied by the log file.
96082536 KB available on disk.
4096 bytes in each allocation unit.
30719999 total allocation units on disk.
24020634 allocation units available on disk.
I have the entire log if you'd think it'd be helpful, but otherwise, the only errors I saw were about 40 cases of moving bad clusters, e.g.
Windows replaced bad clusters in file 282
of name \Windows\Logs\CBS\CBS.log.
I'll run chkdsk on the other partitions, but the *only* time Windows decides it needs to run an unsolicited chkdsk is after I've booted into Win10 (ie. nothing when I go into Win10, but chkdsk *will* occur when I boot back into win7). On my regular, daily boot into Win7, there are never any problems; I rarely boot into Win10 and it's only after I've done so that the chkdsk occurs. So it doesn't really seem like a physical problem.