Why is my data partition readonly all of a sudden? / Why are my startup scripts unable to write to the data partition?
When the slug first boots up, the data partition is readonly until the disk quota system has scanned it and computed the disk usage of all the users. Depending on how much data there is on your data partition, this may take several seconds, or several minutes. Note that this is the default behaviour of Unslung; it will happen even if you have not done anything to configure the quota system.
This issue frequently bites people who have linked /opt to a directory in the data partition in order to get more space for packages. If you do that, you must either disable quotas (see below) or live with the fact that /opt might not always be writable.
If you have no interest in quotas, you might want to disable them by creating a DiversionScript. Placing this content in /unslung/rc.quota should do the trick: