A couple quick things to try (maintenance-related) before jumping deeper into troubleshooting:
The MFT (Master File Table) can get a glitch in it, which can often be repaired by a check disk utility (Windows has one embedded). In Windows Explorer, right click the drive and select "Properties," then on the "Tools" tab select "Check Now" in the Error-checking section. Often this will require a reboot and the check/repair will occur during the next startup cycle. Orphan files (ones not referenced properly in the MFT) cause the disk to actually search manually and can be painfully slow.
Another thing is fragmentation. Drives will write to the first space they find closest to current head position. This speeds up writing, but can make reading a nightmare if piece-parts are all over the disk. A disk defrag is good to try, especially one which "optimizes" file structure. That option puts "related" files closer to each other to accelerate access times but is a LONG defrag to perform. It is best to start that one before going to bed, as it can take a few hours to complete, but is worth doing *if* the files in question will remain relatively static... meaning that they will not be altered often and potentially fragmented again in short order. The "normal" defrag is quicker and can be effective on data files that get modified frequently.