Hi Adam,
thank you very much for your help. I have already tried to run Windows tools: chkdsk and Disk Defragmenter. This is the output of chkdsk:
The type of the file system is NTFS.
WARNING! F parameter not specified.
Running CHKDSK in read-only mode.
CHKDSK is verifying files (stage 1 of 3)...
1369344 file records processed.
File verification completed.
2677 large file records processed.
0 bad file records processed.
2 EA records processed.
65 reparse records processed.
CHKDSK is verifying indexes (stage 2 of 3)...
1770200 index entries processed.
Index verification completed.
0 unindexed files scanned.
0 unindexed files recovered.
CHKDSK is verifying security descriptors (stage 3 of 3)...
1369344 file SDs/SIDs processed.
Security descriptor verification completed.
200429 data files processed.
CHKDSK is verifying Usn Journal...
36604448 USN bytes processed.
Usn Journal verification completed.
Windows has checked the file system and found no problems.
488384535 KB total disk space.
390636096 KB in 1153801 files.
571712 KB in 200430 indexes.
0 KB in bad sectors.
1490615 KB in use by the system.
65536 KB occupied by the log file.
95686112 KB available on disk.
4096 bytes in each allocation unit.
122096133 total allocation units on disk.
23921528 allocation units available on disk.
I also run Disk Defragmenter to consolidate fragmented files on my computer's hard disk to improve system performance (this process took about 12 hours to terminate all steps).
By the way, from time to time I have to wait very long time (more than 30 minutes!) when rebooting my system after installing some Windows updates ( e.g. the last one causing this problem was: "Security Update for Windows 7 for x64-based Systems (KB3011780)" ). This behaviour does not happen whenever rebooting the system but only after installing Windows updates (not all the updates).
I'm not sure but the problem could be related with SED v.10.3.0.9307 (I do not remember if the first time this problem occurred on my system was before or after August 2014 when I upgraded the Symantec product to this version as requested by my company). By the way, in this moment my company's security policy does not allow to change this version... Do you have some evidence of this (i.e. it is a known issue, other customers have similar problems, ...)?
Do you have any other suggestions (e.g. decrypt, defragment and re-encrypt the disks,...)?
Thanks!
Rai