I want to use a USB to store excel files that I can open and edit using an ipad

You may want to look into one of these solutions. Apple does not provide a means of attaching a flash drive to an iDevice and access office files. You must use a third party solution. Apple does provide a means of access picture files.

Airstash
"Easily access, transfer, and stream your files from AirStash® – the pocketable wireless flash drive for iPad, iPhone, and iPod touch! This app requires an AirStash device in order to work. View, import & export photos, wirelessly stream videos, music & podcasts, and view documents stored on your AirStash. AirStash+ is an enhanced alternative to the built-in web interface adding features only possible with a native app."
http://www.airstash.com/

SanDisk
" SanDisk Connect Wireless Stick is the flash drive reinvented to work with both your computer and your mobile devices."https://www.sandisk.com/home/mobile-device-storage/connect-wireless-stick

Seagate Wireless portable hard drive
"The new 500GB Seagate Wireless portable hard drive is the company's latest HD designed specifically for use with tablets and smartphones like Apple's iPad and iPhone."
http://www.macrumors.com/2015/01/04/seagate-ces-2015-new-products/

Allows you to access your own sd card and usb device

http://www.amazon.com/Dane-Elec-Media-Streamer-Mobile-Junkie/dp/B00IA8LLUW

suggested by seeblue

http://www.dpreview.com/forums/thread/3516024

Camouflage data files as a raw image.
read files from a flash drivehttps://www.youtube.com/watch?v=m5f1VmjjczQ

Kingston Digital MobileLite Wireless Flash Reader for Smartphones and Tablets (MLW221)
Kingston Digital MobileLite Wireless Flash Reader for Smartphones and Tablets (MLW221)

CloudFTP
https://www.kickstarter.com/projects/danielchin/cloudftp-wirelessly-share-any-us b-storage-with-ipa

https://www.amazon.com/HyperDrive-iUSBport-CloudFTP-Storage-Wireless/dp/B008U9GS 18

FlashAir™ II Wireless SD Card series

--Share photos & videos wirelessly

--Capacity 16GB

--Supports simultaneous access from up to seven devices

http://us.toshiba.com/accessories/wireless-sd/16gb/class-10/PFW016U-1CCW

Photofast's iFlash Drive
Here is a nice adapter. You can move data in both directions! Be sure to get the real thing.
http://www.photofast.com

review:
http://www.macworld.com/article/2036911/review-i-flashdrive-hd-a-flash-drive-for -ipads-iphones-and-laptops.html

Here is the ios app:
https://itunes.apple.com/us/app/i-flashdrive-hd/id525386291?mt=8

The app get some bad reviews. Turns out there is a cheap imitation product that didn't develop their own software and parasites of of PhotoFast's software. Could be the folks using the imitation product are complaining.

Well, I do not know about the memory limit. It's fairly large. The complaints have been about rather large files. Email the spreadsheet to yourself. Go to an apple store. Download and try. There is a free version of excel for the ipad. The pro excel is paid. Try on a regular ipad.

You will need to use a separate app to access the external flash drive. I have not tried these devices Buy something you can return.

Why do you want to keep the data on an external device? Good for backup, but the iPad is rather secure. FBI paid over a million dollars to hack into the iPhone 5c.

Develop a good backup plan. More people loose access to their data because they have forgotten their password and they have not backup. Please write down all your passwords on a piece of paper.

Robert

I use iCloud for backing up my iPhone. That's all. These cloud services are tend to be syncing services so when you delete the file on one device it is deleted in all. I'd keep the files on your ipad. Use a flash drive for backup.

Use a six character passcode & you will be secure.

Robert