Let me explain...

If you take six ADC samples (one byte for each channel), it is pointless saving them if you are going to do a Page Write of upto 64 bytes, because you've only got 6 bytes of valid Data.

You might as well save those samples up (in your Data array) until you've got 60 samples (100mS worth if you're capturing six every 10mS - therefore you'll have acquired 60 samples across 100mS).

Now once you've got 60 bytes of data (bytes 0-59 of the Page), it's worthwhile doing an EEPROM page write. The last four bytes of each page (bytes 60-63) are unused.

Also, out of each set of 6 bytes (samples from the ADC's), byte 6 is only valid every other byte because you only wanted to sample the 6th input every 20mS. Wasting one byte in twelve might seem wasteful, but I get the feeling you're not experienced enough as yet to deal with the concept of an eleven byte 20mS sample, where the first six bytes belong to the first 10mS period, and the remaining five bytes belong to the 2nd 10mS period.

Like I said before, you CAN'T write to any more than ONE EEPROM AT A TIME, because it'll take 10mS to access each one.

The way we get around this is that although we need to store 650 pages of captured data (remember each page is 60 bytes long, comprising of ten sets of 6 samples, each set being captured every 10mS - therefore each EEPROM page is 100mS worth of data, and 650 pages equals the target 65 seconds you require), we save the first 512 pages sequentially into EEPROM 1, and the remaining 148 pages sequentially into EEPROM 2.

That way, we only ever access ONE EEPROM at a time, so our write delay will only be 10mS (ie Pages 0-511 get written to EEPROM 1, whilst pages 512-649 get written to EEPROM 2).