More information is needed. If H drive is a card reader(for lets say a memory card), it will disappear when there is not a card inserted on Windows 7

If it is a hard drive or partition, you will need to look in disk management and see if it is just not assigned a letter.

In rare cases a card reader will steal a hard drives letter and cause an issue that a simple disk management drive letter assignment will fix.

If H is an external drive. disk management will show if it is detected as well.

Need help with Disk Management? Let me know i will help you further.