Aluratek AEBK07FS - Manual (Page 9)

Each ADE ID supports only 6 devices; this includes the master device
(AEBK07FS) and the slave device (SD card).
When AEBK07FS is restored to default settings, ALL DRM activation
record and data (including DRM books purchased) will be erased from
AEBK07FS. You are required to authorize the device with your ADE ID
again.
When the number of authorized device has exceeded the allowable
amount, you will come across the following error message:
To request for extension and reset of your ADE ID, go to
http://www.adobe.com/support/digitaleditions/  [Submit a web
case]. You are required to key in your ADE ID info.
Notes in using ADE
E_ACT_TOO_MANY_ACTIVATIONS