Is the 64 MB card an MMC card? Also some old cards don't comply with the SD spec.
SD.h only supports SD cards that comply with the SD standard. Macs and Windows machines support old cards but SD.h does not since this would increase the size of the library and few people use cards produced before the standard was finalized.
Also the SD standard requires 64MB cards to be formatted FAT12. SD.h does not support FAT12 file systems.
How did you format the cards? If you used Mac or PC utilities, the card may be formatted with a GUID partition table instead of MBR. PCs and Macs accept GUID partition tables but SD.h does not.
The SD standard requires a MBR partition table. The best way to format SD cards is to use SdFormatter https://www.sdcard.org/downloads/formatter_3/.
It would be helpful if you downloaded SdFat Google Code Archive - Long-term storage for Google Code Project Hosting. and ran the QuickStart and SdInfo examples and posted any error codes from these programs. These programs are in the SdFat/examples folder.
You can also force the 64 MB card to be formatted FAT16 by using the SdFormatter example in the SdFat/examples folder.