I have a Windows 7 PC with 2 physical hard disk drives on it. The first drive has 2 partitions (C, D) and the second also has 2 partitions (E, F). OS is installed on C. Recently I started noticing a strange behavior. 3 of the 4 partitions (D,E,F) become unavailable (not visible in My Computer, and I am not able to access them by entering an address like D:\ in windows explorer). However, the drives become available again once I restart the system only to disappear again after some hours of use. Both drives also show up in BIOS. I am guessing it is not an issue with the physical drive since partitions on both physical drives are affected. I have done a drive scan and a virus scan using Symantec Endpoint Security on all drives without much luck. Any help in resolving this issue will be much appreciated, please.