I got a new asus laptop for christmas today and when i go to open up my Toshiba external hard drive on that computer, i can't seem to find it anywhere. It opens up on my Macbook Air (which is where i used it first if that matters) but not on my new asus. Is there any way that i can fix that?
Did you format it for Mac OS X (HFS+ Journaled)? That is unreadable by Windows.
You can either buy a utility to read/write HFS+ in Windows. Such as Paragon HFS+ or MediaFour Macdrive. Which is what you will have to do if you use the drive for Time Machine.
The free option is to format it using a filesystem natively supported by Windows and Mac OS X. Either use FAT32 or ExFAT. I would use ExFAT due to the 4GB file size limitation of FAT32. This breaks Time Machine support (unless Apple changed that). You will likely have to use Disk Utility in Mac OS X to do the format. Window often balks when you try to format a GPT disk originally formatted by a Mac.
In addition to what @velocityg4 mentioned, I'd strongly recommend you back up all your data from the external drive on the Macbook, since you are able to access it from there. After making a duplicate of the files stored on the external HDD, you would be safe to re-format it in exFAT or FAT32 as it was already suggested. You could do this on the Macbook as well by Re-formatting the HDD through Disk utility or you could do it through the Windows laptop in Disk management. However, I'd strongly advise you to check some of the File & Partition size limitations that come with the FAT32 file system.