The most likely reason a second system would not be able to see the files stored on the drive would be due to some incompatibility of the second system with the file system of the drive. For example, if the first system was Windows 8, but the second system was OS X, OS X is unable to read partitions formatted with NTFS. Another possible incompatibility would be between drives partitioned with MBR and GPT. For example, if the first system was Windows 8, but the second system was Windows XP, Windows XP is unable to read drives partitioned with GPT.
The solution in either case would be to back up the data from the drive, reformat it with a compatible file system or convert it to a compatible partitioning technology.
These operations can be performed with Disk Management, which in Windows 8 can be accessed quickly by right clicking in the lower left corner, where the Start Screen would be launched, then selecting Disk Management from the list of administrative tools.
You may also want to read more about GPT to discover more about the technology, why it is used, and how it affects compatibility in various scenarios.
There is a list of some very good questions found on MSDN in the Windows and GPT FAQ.
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